1. Field of the Invention
The present invention relates to a developing bias control system for controlling a developing bias in an image forming apparatus such as a copier or a laser printer.
2. Description of the Related Art
In a conventional image forming apparatus, the surface of a photosensitive member uniformly charged by a charging unit is exposed by using an exposing unit such that an electrostatic latent image is formed thereon. The electrostatic latent image is then changed to a visible image with a toner and the resulting toner image is transferred onto a transfer material. As a developing method adopted in the case of using a magnetic single-component developing scheme in a developing process in which the electrostatic latent image is changed to a visible image with a toner, there is a non-contact developing method whereby development is conducted by allowing a toner to fly between the surface of the photosensitive member and a developing apparatus for supplying the toner.
FIG. 7 is a view for illustrating the non-contact developing method. In the drawing, 901 denotes a developing sleeve, 902 denotes a magnet roller, and 903 denotes a magnetic blade. In accordance with the non-contact developing method, a toner on the developing sleeve 901 is kept from contact with a photosensitive drum 905 and development is conducted by allowing the toner to fly in the space therebetween (hereinafter referred to as "developing gap"), as shown in the drawing.
Specifically, the toner is triboelectrically charged by means of the developing sleeve 901 and the magnetic blade 903 to form a thin-film toner layer on the developing sleeve 901. Thereafter, the toner layer is brought closer to the surface of the photosensitive drum 905 by the rotation of the developing sleeve 901. In the mean time, a developing bias is applied from a developing bias power source 904 to allow the toner to fly in the developing gap, whereby development is conducted. As the developing bias, a developing bias voltage obtained by superimposing a direct-current (hereinafter referred to as "DC") voltage on an alternating-current (hereinafter referred to as "AC") voltage is applied. In this case, the upper voltage time rate of the AC voltage or the lower voltage time rate thereof is usually set to 50%.
Here, "upper voltage time rate or lower voltage time rate" is defined as a rate, of one period of the AC component, constituted by a time during which the AC voltage has a value higher or lower than a reference voltage. The reference voltage is set such that an area occupied by a portion of one cycle of the signal waveform of the AC voltage in which the AC voltage has a value higher than a given voltage (hereinafter referred to as "area middle voltage") as the reference voltage becomes equal to an area occupied by a portion of the signal waveform in which the AC voltage has a value lower than the given voltage.
A description will be given to the waveform in the case of applying the developing bias voltage obtained by superimposing the DC voltage on the rectangular-wave AC voltage.
FIG. 8 is a view diagrammatically showing the signal waveform of the developing bias voltage in the case of superimposing the DC voltage at -250 V on the rectangular-wave AC voltage at a 50% upper or lower voltage time rate.
As shown in FIG. 8, the developing bias voltage has a value of -250 V in the middle of the upper and lower peak voltages (hereinafter referred to as "V.sub.p-p middle", in which p-p stands for "peak to peak"). On the other hand, the voltage in the middle (hereinafter referred to as "area middle") at which the area of the region surrounded by the peak portion of the AC voltage waveform (area of the hatched portion A in the drawing) coincides with the area of the region surrounded by the valley portion thereof (area of the hatched portion B in the drawing), i.e., the area middle voltage mentioned above also becomes -250 V.
As described above, if the upper voltage time rate or the lower voltage time rate is 50%, the voltage in the V.sub.p-p middle and the area middle voltage are equal to each other and to the superimposed DC voltage.
To obtain a proper solid density by controlling a developing density in the case of using the non-contact developing method, it is preferable to freely control the upper or lower voltage time rate of the rectangular-wave AC voltage applied as the developing bias and a voltage range between the upper and lower peak voltages (hereinafter referred to as "upper-to-lower voltage range" or "V.sub.p-p "). This is because the non-contact developing method is, namely, a developing method utilizing the reciprocal movement of the toner in the developing gap caused with the application of the AC voltage so that the upper or lower voltage time rate or the value of V.sub.p-p affects such reciprocal movement of the toner.
If the upper or lower voltage time rate is controlled to be other than 50%, however, the following problems occur. FIG. 9 is a view for illustrating the problems.
As the developing bias, it is normally preferred to cause the voltage in the V.sub.p-p middle of the rectangular-wave AC voltage to coincide with the surface potential of the photosensitive member or the like (in the example of FIG. 9, the surface potential of the photosensitive member or the like is assumed to be -250 V).
The reason for this is that, in principle, V.sub.p-p is preferably maximized in the non-contact developing method. Specifically, the problem of degraded halftone reproduction occurs if V.sub.p-p can be set only to a value smaller than required.
If the upper and lower peak voltages exceed a discharge limit as a result of increasing V.sub.p-p, on the other hand, a discharge occurs in the developing gap, which degrades image quality. To maximize V.sub.p-p such that the discharge limit is not surpassed, therefore, it is preferable to cause the voltage in the V.sub.p-p middle to coincide with the surface potential of the photosensitive member.
If the upper or lower voltage time rate is other than 50%, however, the problem occurs that the DC voltage is not necessarily a voltage in the V.sub.p-p middle even when the DC voltage to be superimposed is caused to coincide with the potential at the surface of the photosensitive member. This is because, in accordance with the principle of the AC transformer, the DC voltage to be superimposed becomes the area middle voltage, not the voltage in the V.sub.p-p middle. If the DC voltage to be superimposed is controlled to coincide with the potential at the surface of the photosensitive member, therefore, there are cases where the peak value of the rectangular-wave AC voltage exceeds the discharge limit. The problem encountered when V.sub.p-p is reduced such that the discharge limit is not surpassed is as described above.